diff --git a/src/plugins/extras/README.md b/src/plugins/extras/README.md
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+Extras Plugin
+=============
+
+The Extras plugin will initialize the optional addon plugins from 
+[extras/](../../../extras/) directory, if they were loaded.
+
+Generally, for an extras plugin to have been loaded, 2 things must have happened:
+
+1. The extras plugins must be present in extras/ directory, for example after 
+   running `git submodule update`
+2. One or more extras plugins are added to the impress.js presentation (the HTML
+   file) by the author using a regular `<script>` tag.
+
+If one or more extras plugins were so added, this plugin will automatically
+discover them and perform initialization (such as calling 
+`mermaid.initialize()`).
+
+If no extras plugins are added to a presentation, this plugin does nothing.
+
+Note that some extra plugins (like mathjax) initialize themselves immediately, and
+there's nothing to do here.
+
+Author
+------
+
+Henrik Ingo (@henrikingo), 2016
diff --git a/src/plugins/extras/extras.js b/src/plugins/extras/extras.js
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+/**
+ * Extras Plugin
+ *
+ * This plugin performs initialization (like calling mermaid.initialize())
+ * for the extras/ plugins if they are loaded into a presentation.
+ *
+ * See README.md for details.
+ *
+ * Copyright 2016 Henrik Ingo (@henrikingo)
+ * Released under the MIT license.
+ */
+/* global markdown, hljs, mermaid, impress, document, window */
+
+( function( document, window ) {
+    "use strict";
+
+    var preInit = function() {
+        if ( window.markdown ) {
+
+            // Unlike the other extras, Markdown.js doesn't by default do anything in
+            // particular. We do it ourselves here.
+            // In addition, we use "-----" as a delimiter for new slide.
+
+            // Query all .markdown elements and translate to HTML
+            var markdownDivs = document.querySelectorAll( ".markdown" );
+            for ( var idx = 0; idx < markdownDivs.length; idx++ ) {
+              var element = markdownDivs[ idx ];
+
+              var slides = element.textContent.split( /^-----$/m );
+              var i = slides.length - 1;
+              element.innerHTML = markdown.toHTML( slides[ i ] );
+
+              // If there's an id, unset it for last, and all other, elements,
+              // and then set it for the first.
+              var id = null;
+              if ( element.id ) {
+                id = element.id;
+                element.id = "";
+              }
+              i--;
+              while ( i >= 0 ) {
+                var newElement = element.cloneNode( false );
+                newElement.innerHTML = markdown.toHTML( slides[ i ] );
+                element.parentNode.insertBefore( newElement, element );
+                element = newElement;
+                i--;
+              }
+              if ( id !== null ) {
+                element.id = id;
+              }
+            }
+        } // Markdown
+
+        if ( window.hljs ) {
+            hljs.initHighlightingOnLoad();
+        }
+
+        if ( window.mermaid ) {
+            mermaid.initialize( { startOnLoad:true } );
+        }
+    };
+
+    // Register the plugin to be called in pre-init phase
+    // Note: Markdown.js should run early/first, because it creates new div elements.
+    // So add this with a lower-than-default weight.
+    impress.addPreInitPlugin( preInit, 1 );
+
+} )( document, window );
+